Aircraft skin panels typically are joined to underlying stringers or other supports by flush head fasteners, such as rivets. It is desirable that exposed surfaces of the skin panels be smooth for aerodynamic efficiency, for increased adhesion and uniform application of protective films and paints, and for aesthetic purposes. A glossy, smooth painted surface is more pleasing to the eye than a dimpled one.
In a conventional flush head fastener installation system, the holes for receiving rivets have frustoconical countersinks in the skin panel to receive matching fasteners having frustoconical surfaces under the heads. A perfect knife edge at the periphery of the fastener head is not possible; it necessarily is rounded. The rounded periphery of the fastener head, as compared to the frustoconical countersink, results in an annular groove or "halo" surrounding the fastener head.
For example, with reference to FIG. 1, in a known riveting system the included angle .alpha. of the underside of the head portion 10 of a rivet R is 100.degree., and the countersink 12 is cut in the skin panel 14 to match. The shank 16 of the rivet is cylindrical and, prior to formation of the upset 18, has a diameter D less than the corresponding diameter of the drilled hole 20 cut in the stringer or other support member 22 and the lower portion of the skin panel. Drilled hole 20 meets the countersink 12 at a corner 24 which typically is located closer to the abutting surfaces 26 of the outer skin 14 and underlying support 22 than to the exterior surface 28 of the skin. Prior to formation of the upset 18, the rivet can have a dome head, represented by the broken line 30 in FIG. 1, and the shank 16 is of substantially uniform circular cross-section as represented by broken lines 32. When the rivet is crushed by pressure applied by oppositely disposed anvils, the shank 16 of the rivet expands, the domed head 30 is flattened, the upset 18 is formed, and the skin panel 14 is clamped to the support 22. As noted above, since the rivet head cannot be formed with a perfect knife edge, the maximum diameter E of the rivet head is less than the diameter E' of the countersink 12 at the location where it opens at the exterior surface 28 of the skin panel. An annular groove or halo 34 of a depth F and width G is formed around the rivet head.
Dimensions F and G are at least partly functions of permissible tolerances pertaining to the rivet head at the time of manufacture. In the past, one way in which the halo problem has been attacked is to require rivet manufacturers to produce rivets with a rivet head height F of smaller and smaller dimensions. This increases the cost of the rivets, and practical limits have been reached without a completely satisfactory solution to the halo problem.
An alternative geometry disclosed in U.S. Pat. No. 4,000,680 is shown in FIG. 2 and FIG. 3. Skin panel 14 is joined to a stringer or underlying support 22 by the modified rivet R' which, prior to being crushed by the oppositely disposed anvils A, has a substantially cylindrical shank 16 of a diameter D less than the diameter D' of the hole 20 through the support 22 and lower portion of skin panel 14. As for the riveting system of FIG. 1, the rivet head has a frustoconical underside, and the countersink 12 in the skin panel is cut at an identical angle. However, in the geometry of FIG. 2 and FIG. 3, the depth F of the rivet head from the frustoconical underside to the exterior (top) of the head is deliberately formed as a cylinder coaxial with the rivet shank 16. In addition, rather than using a simple domed top, the rivet head is formed with an annular "ring dome" 36 with a depression 38 at the center. The more complicated countersink is formed by a cutter of the type shown in FIG. 4. The cutter has a nose portion 40 of a diameter D' larger than the diameter D of the shank of the rivet with which the cutter is intended to be used. An angled, frustoconical portion 42 of the cutter transitions from the diameter D' of the nose portion to the larger diameter E' of a larger cylindrical portion of the cutter.
Returning to FIG. 2, the diameter E of the rivet head is slightly smaller than the diameter E' of the outer cylindrical portion of the countersink. As seen in FIG. 3, when the rivet is crushed to form the upset 18 by the opposing anvils A, the shank 16 of the rivet expands, and the ring dome is flattened, forcing the rivet head outward to substantially fill the countersink.
The riveting system of FIGS. 2 and 3 has been reasonably effective in eliminating or at least reducing rivet haloes. However, the system is expensive, due to the complicated geometry, tight tolerances, fastener type and license fees. One reason for the tight tolerances is that an additional stress point is added at the junction 44 of the outer cylindrical portion 46 of the countersink and the frustoconical portion 48. As currently used, this junction is located at least 0.014 inch below the exterior surface of the skin panel (for shank diameters of 0.125") and even deeper for rivets of increasing diameter. For example, for rivet diameters nominally 0.312 inch (10/32") the junction 44 is at least 0.033 inch below the exterior surface of the skin panel for systems currently in use.